RadioAstron orbit determination and evaluation of its results using correlation of space-VLBI observations

Abstract

A crucial part of a space mission for very-long baseline interferometery (VLBI), which is the technique capable of providing the highest resolution images in astronomy, is orbit determination of the missions space radio telescope(s). In order to successfully detect interference fringes that result from correlation of the signals recorded by a ground-based and a space-borne radio telescope, the propagation delays experienced in the near-Earth space by radio waves emitted by the source and the relativity effects on each telescopes clock need to be evaluated, which requires accurate knowledge of position and velocity of the space radio telescope. In this paper we describe our approach to orbit determination (OD) of the RadioAstron spacecraft of the RadioAstron space-VLBI mission. Determining RadioAstrons orbit is complicated due to several factors: strong solar radiation pressure, a highly eccentric orbit, and frequent orbit perturbations caused by the attitude control system. We show that in order to maintain the OD accuracy required for processing space-VLBI observations at cm-wavelengths it is required to take into account the additional data on thruster firings, reaction wheel rotation rates, and attitude of the spacecraft.We also investigate into using the unique orbit data available only for a space-VLBI spacecraft, i.e. the residual delays and delay rates that result from VLBI data processing, as a means to evaluate the achieved OD accuracy. We present the results of the first experience of OD accuracy evaluation of this kind, using more than 5,000 residual values obtained as a result of space-VLBI observations performed over 7 years of the RadioAstron mission operations.

The gravitational redshift monitored with RadioAstron from near Earth up to 350,000 km

“…Figure 6: Estimates of for each downlink carrier frequency computed using ∆z b obs from each pair of 1 h sessions where |∆z| min = 1 × 10 −11 and ∆t max = 4.5 d. Each point is plotted at the mid-point of the session pair. The rms scatter of 0.07 at PU and 0.08 at GB, is ∼ 3 times smaller than what would be expected from the uncertainty due to errors in the orbital velocity of the spacecraft found to be 1.36 mm/s (Zakhvatkin et al, 2018) in any direction. The parabolic fits, almost indistinguishable at the two downlink carrier frequencies, highlight a trend which we consider to be a systematic error.…”

“…To look for a possible violation using measured fractional frequency shifts, the state parameters for the spacecraft must be known with sufficient accuracy to isolate the gravitational redshift from other effects, particularly the much larger non-relativistic Doppler effect. Orbit determination for the RadioAstron mission is done using a variety of sources of data, among them the same frequency observations made at the PU and GB ground stations 6 used in this analysis (Zakhvatkin et al, 2013(Zakhvatkin et al, , 2018. The position and velocity of the spacecraft are determined using a least-squares minimization in which no violation is assumed.…”

Testing the Einstein equivalence principle with two Earth-orbiting clocks

“…For our analysis we pick the time segment of 01/01/2014-01/01/2015, which starts in the low perigee epoch (perigee altitude ∼1000 km) and ends in the high perigee epoch (respectively, ∼50 000 km). We use the long-term predicted orbit provided for the RadioAstron mission by the ballistic center of the Keldysh Institute for Applied Mathematics (KIAM) [22]. For the ground station we choose the mission's Green Bank Earth Station with the geodetic latitude of +38 • 26 16.166 , longitude of −79 • 50 08.810 , and height of 812.50 m [23].…”

The antenna phase center motion effect in high-accuracy spacecraft tracking experiments

“…However, it can easily fail when tracking SC. Indeed, the accuracy of predicted orbits used for tracking SVLBI spacecraft can be as low as 5 km for each component of the position vector, which corresponds to ∠(ŝ, ŝ ) ∼ 17 for the distance to the SC of ∼ 1, 000 km (Zakhvatkin et al, 2020). When a ground TS is receiving signals sent by a SC, its antenna operators usually can partially compensate for the inaccuracy of the predicted orbit by applying constant pointing offsets that maximize the level of the received signal.…”

“…In our analysis we use long-term predicted orbits provided for the RadioAstron mission by the ballistic center of the Keldysh Institute for Applied Mathematics (KIAM) (Zakhvatkin et al, 2020). Orbits of this type were produced primarily for the purpose of long-term planning of the mission operations.…”